An aliphatic polyester, such as polyglycolic acid or polylactic acid, can be decomposed by microorganisms or enzymes present in nature, such as soil or sea water, and is therefore noted as a biodegradable polymer material giving little load to the environment. Further, an aliphatic polyester has degradability and absorbability in vivo, so that it is used also as a polymer material for medical use, such as sutures for surgery or artificial skin.
Among the aliphatic polyesters, polyglycolic acid is excellent in gas barrier properties, such as oxygen gas barrier property, carbon dioxide gas barrier property and water vapor barrier property, and also is excellent in heat resistance and mechanical properties, and therefore the development of new use thereof by itself or as a composite material together with another resin material, is under way.
However, aliphatic polyesters are generally hydrolyzable, and particularly polyglycolic acid resin has a remarkable tendency thereof and involves problems that its barrier property and strength are lowered along with its hydrolysis.
There have been individually known that the hydrolyzability of an aliphatic polyester inclusive of polyglycolic acid resin is related with a terminal carboxyl group concentration and the moisture resistance of an aliphatic polyester can be increased by adding a carboxyl group-capping agent (Patent document 1 listed below), and the hydrolyzability of polyglycolic acid resin is related with a residual glycolide content (Patent document 2 below). On the other, it has been also known that a resistance to thermal resistance of polylactic acid around its melting point can be increased by adding a polymerization catalyst-deactivation agent thereto (Patent document 3 below). However, there has not been known an influence of a carboxyl group-capping agent on a moisture resistance (more specifically, a resistance to decomposition with water under a lower temperature condition of e.g., around room temperature, or in a hydrolysis-promoting environment of, e.g.,. 50° C. and 90%-relative humidity) of polyglycolic acid resin.
Patent document 1: JP2001-261797A,
Patent document 2: W02005/090438A,
Patent document 3: JP9-124778A,
Patent document 4: WO2007/060981A